Heated and cooled mixture of wax and polyethylene



United States Patent 3,165,823 HEATED AND CGOLED MDKTURE 0F WAX AND POLYETHYLENE Arthur H. Boenau, Flushing, N.Y., assignor to Socony Mobil Gil Company, Inc., a corporation of New York No Drawing. Griginal application dune 11, 1958, Ser. No. 741,240, new Patent No. 2,99,765, dated Sept. 12, 1961. Divided and this application Mar. it), 1960, Ser. No. 13,981

2 Claims. (Cl. zeta-ass This invention is related to a method of coating milk containers and the product formed thereby. The invention particularly relates to an improved paper coating applied in two steps with a substantial reduction in wax requirement.

A large variety of wax compositions are used commercially for coating paper and paperboard. For example, parafiin wax having a melting point between about 120'- 150 F. has been used extensively and this has been improved by the addition thereto of various addition agents, such as polyethylene and microcrystalline wax. A large portion of the wax coated board is used in the milk industry to replace glass bottles, and with the rapid growth of this business the requirement for coating wax is constantly increasing.

The paraflin wax is obtained from selected petroleum waxy distillates which are chilled and mixed with suitable solvents such as liquefied propane, petroleum naphtha, methyl ethyl ketone, toluene, etc., or combinations thereof. The mixture is filtered to remove the wax from the oil and the solvent is removed from the wax. This wax is termed slack wax and still contains about -40 percent of oil. In some cases slack wax is also obtained from the cold-pressing of wax distillate per se. The slack wax is then treated to effect further removal of oil, either in conventional sweating operations or by further solvent extraction, producing a crude scale wax having an oil content of about 1-3 percent. By more precise re-sweating operations or crystallization from suitable solvents, the wax is converted to a semi-refined wax having an oil content of 0.5-1 percent or a fully refined wax having an oil content of less than 0.5 percent, usually 0.1-0.3 percent. The presence of any substantial amount of oil in the wax is detrimental to the tensile strength of the wax and hence the fully refined Wax is particularly required for coating board used as milk containers.

In contrast to paraffin wax, microcrystalline wax is obtained from solvent-extraction operations on the residual portion of crude petroleum. Usually the residuum is first subjected to a deasphalting treatment to permit proper crystallization of the wax in the subsequent chilling of the mixture with solvent. After filtering, the petrolatum obtained is mixed with an additional quantity of solvent; the mixture is chilled and filtered to remove the microcrystalline wax and the solvent is removed from the wax. Microcrystalline Waxes have melting points usually in the range of 145-175 F., and their viscosities are about 12 20 centistokes at 210 F., whereas the viscosities of parafiin waxes are about 3-6 centistokes at 210 F.

There are two methods currently used in applying wax to paperboard containers.

A. The preformed carton is dipped in molten wax, drained, cooled and shipped to dairies ready for filling with milk, cream, etc.

B. Many dairies are equipped with machines which continuously perform a series of operations. Flat cartons are introduced into the machine, which forms each carton and glues the bottom flaps. The carton is then dipped in molten wax, drained and cooled. The waxed carton is then filled with milk, cream, etc.

3-,l5,823 Fatented Oct. 1, 1963 In both cases, the wax is applied in a single-dip operation.

A large portion of the coating wax is absorbed in the paperboard during the single-dip operation and hence a substantial amount of wax is required to provide an adequate wax coating giving full protection against leakage. I have discovered that this large wax requirement can be substantially reduced by applying wax in two separate operations. The first is one of controlled impregnation of the paperboard with a parafiin wax in which has been blended a substantial amount of an ethylene polymer. Surface coating is eliminated in this operation by wiping the paperboard as it emerges from the liquid wax-polymer blend. The second step is then conducted by either of the two methods previously described for the single-dip operation, using parafiin wax containing little or no ethylene polymer. The combination of these two steps in applying wax results in the aforementioned substantial reduction of wax requirement by virtue of decreasing the amount of impregnation to one-half or less of that from the single-dip operation, while holding the amount of surface coating nearly the same.

It is accordingly an object of the present invention to provide a method of preparing a suitable impregnant for milk-container paperboard. Another object is to provide a two-step operation for applying Wax to milk containers. A still further object is to afford a means whereby the number of cartons treated in accordance with the present invention can be substantially increased over those waxed by the current single-dip operations from the same quantity of wax. These and other objects which will be recognized by those skilled in the art are attained in accordance with the present invention, wherein wax is applied under prescribed conditions in two separate operations.

For this first step, however, the ethylene polymer must be completely in solution with the wax to yield the desired results. It is commonly known that ethylene polymers are difficult to get into solution with wax and for this reason only very limited amounts of ethylene polymers have been used to improve the coating wax. Efforts to bring a substantial amount of polyethylene into solution with a wax have always resulted in the formation of a precipitate of the polymer on the cooling of the solution. I have discovered that this can be avoided by heating the polyethylene and wax, first, to a temperature above the melting point of the polyethylene or at least close to it, second, by cooling the mixture to a temperature below the cloud point of the mixture but above the melting point of the wax, third, reheating to a temperature above the cloud point of the mixture but below a critical maximum temperature found to be a point substantially below the melting temperature of the polyethylene. I have found that if this sequence of steps is followed, the polyethylene-Wax structure is homogeneous and, upon cooling below the cloud point of the mixture, separation into two phases does not occur. In fact, if the mixture is allowed to cool to a temperature well below the cloud point, but above the melting point of the mixture, it forms a gel structure. In this form, the mixture can be reheated to a suitable temperature for application without destroying its efiectiveness. This method of mixing polyethylene with wax provides a simple technique for providing a suitable impregnating solution and can be applied to a large variety of waxes and polyethylenes. The method can be used, however, only where a substantial amount of polyethylene is blended with the wax. The lower limit of polyethylene is about 2 percent or the blend, although this cut-oil point is not sharply defined. The lowest percent of polyethylene will vary to some extent with different polyethylenes and with difierent waxes but is in 3 the neighborhood of 2-3 percent by weight of the mixture.

As an illustration of the criticality of the content of polyethylene in the blend in order to practice the teaching of this invention, a polyethylene (Epolene N as sold cornmerically today by Eastman Chemical Products, "inc, of Kingsport, Tennessee), was blended with a paraffin wax of approximately 128 F. melting point by heating and stirring until a maximum temperature of 225 F. was reached. On cooling to approximately 150 F., the polyethylene precipitated out of solution. This procedure was followed with mixtures of varying polyethylene content and the mixtures were then reheated to a temperature just sufficient to obtain a clear solution, the precise temperature varying with the polyethylene content, and the mixtures were then allowed to cool to 150 F. A gel structure was found to exist at 3 percent or greater polyethylene content, between 2-3 percent the mixture was soupy but homogeneous. A-t 1-l.5 percent polyethylene the blends were fluid and cloudy, but the cloud appeared uniformly throughout each mass. On the other hand, at 0.5 percent, 0.1 percent and 0.05 percent polyethylene, the polyethylene precipitated out of solution. In order to provide the desired intimate blending of polyethylene and wax required for this invention, therefore, the polyethylene content should be at least about 2 percent of the blend. The temperature required for reheating the cooled mixture of polyethylene and Wax to effect removal of the cloud is as follows for the various polyethylene contents indicated.

TABLE I Temperature, F., for

in blend clarity 2 F.) 5 a. 199 4 197 /2 3 197 2 196 195 10 195 05 191 0.1 185 0.05 180 Connnercially available, approximately 2500-3000 molecular weight (Staudinger method).

These tests showed that the blend could not be reheated to a temperature much above the temperatures indicated above without damage to the structure, since when each blend was reheated to 210 F. or higher and then allowed to cool to 150 F. (at least some temperature below its cloud point), the polyethylene precipitated out of solution in all cases.

The polyethylene tested hereinabove and reported in Table I had a molecular weight of about 2500-3000. A straight-chain polyethylene of approximately 750 molecular weight, in the amount of 5 percent in the same parailin wax of 128 F. melting point, behaved in the same manner as an equal percentage of a branched-chain polymer having a molecular weight of about 2500-3000 (Standinger method).

In another case, a polyethylene having a molecular weight (Staudinger method) of approximately 20,000 (Alathon l0 sold by E. l. du Pont de Nemours & Co., Inc.) was blended with a parafiin wax in proportion of 2 percent polyethylene. The mixture was heated, with stirring, to a temperature of 335 F. and cooled to- 150 F., at which temperature the polyethylene was precipitated out of solution. The mixture was reheated to 205 F., at which temperature the polyethylene went into solution and the mixture remained homogeneous as in a gel structure on being rocooled to 150 F.

Thus, the polyethylene used may vary quite considerably and may range in molecular weight from about 500- 20,000 (Staudinger method). The lower limit of molecusorted in a hole near one end of the panel.

was allowed to drain for 10 seconds and then it was as about 2 percent or more, the carton stock absorbs enough of this mixture to provide a good base for the final coating wax and to prevent the impregnation of any substantial amount of the coating wax into the board. The impregnated board carrying the polyethylene-wax blend as impregnant is dipped in the coating wax as the second step of the process to provide the finished coating.

Substantially less total wax is applied to the board, but

the coating is entirely adequate for milk containers, providing a satisfactory seal. The first dipping operation can be etlected at the board plant, whereas the second dipping step can be carried out either at the plant which manufactures preformed cartons or at the diary plant equipped as described previously.

This technique provides a cheaper coating and matenially increases the surface area that can be coated with available wax supplies. wax is a secondary product in the petroleum business and the amount available fluctuates to a large extent with the production of other petroleum products. Thus, this two-step coating process provides flexibility whereby the number of containers coated with the wax can be increased in spite of low wax supply, in comparison with single-dip operations.

For the various experiments performed, I made test' panels 5 inches x 2% inches cut from a roll of hmfgallon car-ton stock. These panels were air-conditioned in a room held at 70-72 F. and approximately 50 percent relative humidity. The amounts of impregnation and coating were determined from the weights of the panels before and after dipping. dip operation and in the second step of the two-step process, each panel was dipped in the coating wax, contained in a 600-milliliter, tall-form beaker and held at the desired temperature for 15 seconds, after which the panel was removed by means of a bent paperclipin- The panel inverted to permit drainage of wax in the opposite direction for 25 seconds. The waxed panel was then dipped for 3-5 seconds in cool water to set the coating. This procedure simulates plant operations. In the case of determining the amount of impregnation, as in the first step of this invention, the panels were dipped in the wax-' polyethylene blend for 15 seconds at the desired tcmperature and were'wiped immediately with a cloth to remove the excess of the impregnant.

I found that when the board was dipped for 15 seconds in percent paraffin wax of 128 F. melting point at a temperature of 170 F. and was wiped immediately, the amount of impregnation was 22.8 percent. When the board was dipped for 15 seconds in a blend consisting of 5 percent of a polyethylene having a molecular weight (Staudin-ger method) of 2500-3000 and 95 percent of the same paraflin wax and brought into homogeneity as indicated hereinabove and held at 185 F.,-'

the amount of im regnation was only 8.0 percent. For further comparison, a similar Wax polyethylene blend was formed in solution at 225 F. and then was cooled to F. The polyethylene was precipitated and in suspension. No attempt was made to re-dissolve the precipitate. The board dipped in this medium at 150 F. absorbed 23.2 percent by weight. This is practically the same amount of impregnation as that obtained when dipping the board in 100 percent paraffin wax at F. It shows that the polymer should be in solution in order to build up the viscosity of the liquid wax and'thereby to retard impregnation.

In the case of the single- 0 TABLE -II Wax Consumption Data 1 128 F. M.P. paraflin-S percent polyethylene of 2500-3000 mo1.wt., 185 F 2 8.0 (2) Coating from 100 percent paraflin wax of 128 F. M.P.@170 F 18.6

Total consumption 26.6

By weight of test panels and using test procedures outlined =hereinabove. In first step. In second step. It is seen from the above indicated example that the number of cartons coated could be increased or 1.5 times the present output when two-step operations are substituted for the present single-dip operation.

The first operation is intended to effect a limited penetration of the mixture into the paperboard and provide a base for the attachment of the second coating operation. It is desirable to limit the amount of the first blend on the paperboard, therefore, by Wiping the board after the dipping operation. This can be done at the board plant by continuously drawing the impregnated paperboard into contact with wiping blades mounted to remove the excess liquid wax-polyethylene blend from the exterior of the paperboard and to permit drainage back to the source of application. The paperboard may be dipped into the liquid at various temperatures, such as, for example, 150200 F. It is by fixing the temperature, the composition of the wax-polyethylene blend, and the period of time during which the board is sub merged in the liquid blend, that the amount of impregnais controlled. As stated previously, the second step is then conducted by either of the two methods described for the single-dip operation.

For the initial dipping material, the polyethylene content may vary from about 2 percent to about 10 percent by weight of the finished blend, depending upon the grade of polyethylene. A more preferred range of polyethylene content is between about 3-7 percent by weight of the blend, when using a polyethylene having a molecular weight (Staudinger method) of about 1500-4000.

This case is a divisional application of case Serial Number 741,240, filed June 11, 1958, now US. Patent No. 2,999,765, issued September 12, 1961.

I claim:

1. A method of providing a dipping solution suitable for impregnating milk-container carton stock comprising the steps of combining petroleum wax with about 2-10 percent polyethylene, heating the petroleum wax and polyethylene to at least about the melting temperature of the polyethylene to form a solution of the polyethylene and wax, cooling the mixture below the cloud point to cause the precipitation of a substantial portion of the polyethylene, reheating the polyethylene and wax mixture to a temperature above the cloud point but substantially below the melting temperature of the polyethylene to form a clear solution and reducing the temperature below the cloud point to a suitable dipping temperature whereby a suitable dipping solution is formed free of precipitated polyethylene.

2. A method of providing a dipping solution suitable for impregnating milk-container carton stock comprising the steps of combining petroleum wax with about 3-'7 percent polyethylene, having a molecular weight (Staudinger method) within the range of 1500 to 4000, heating the petroleum wax and polyethylene to at least about the melting temperature of the polyethylene to form a solution of polyethylene and wax, cooling the mixture below the cloud point to cause the precipitation of a substantial portion of the polyethylene, reheating the polyethylene md wax mixture to a temperature above the cloud point but substantially below the melting temperature of the polyethylene to form a clear solution and reducing the temperature below the cloud point to a suitable dipping temperature whereby a suitable dipping solution is \formed .free of precipitated polyethylene.

References Cited in the file of this patent UNITED STATES PATENTS 2,758,100 Bailly et a1. Aug. 7, 1956 2,773,045 Simerl et a1. Dec. 4, 1956 2,808,382 Jakaitis Oct. 1, 1957 2,877,196 Reding Mar. 10, 1959 

1. A METHOD OF PROVIDING A DIPPING SOLUTION SUITABLE FOR IMPREGNATING MILK-CONTAINER CARBON STOCK COMPRISING THE STEPS OF COMBINING PETROLEUM WAX WITH ABOUT 2-10 PERCENT POLYETHYLENE, HEATING THE PETROLEUM WAX AND POLYETHYLENE TO AT LEAST ABOUT THE MELTING TEMPERATURE OF THE POLYETHYLENE TO FORM A SOLUTION OF THE POLYETHYLENE AND WAX, COOLING THE MIXTURE BELOW THE CLOUD POINT TO CAUSE THE PRECIPITATION OF A SUBSTANTIAL PORTION OF THE POLYRETYLENE; REHEATING THE POLYETHYLENE AND WAX MIXTURE TO A TEMPERATURE ABOVE THE CLOUD POINT BUT SUBSTANTIALLY BELOW THE MELTING TEMPERATURE OF THE POLYETHYLENE TO FORM A CLEAR SOLUTION AND REDUCING THE TEMPERATURE BELOW THE CLOUD POINT TO A SUITABLE DIPPING TEMPERATURE WHEREBY A SUITABLE DIPPING SOLUTION IS FORMED FREE OF PRECIPITATED POLYETHYLENE. 